This invention relates to an image forming method and an image forming apparatus for ejecting and hitting charged matter onto a record medium for forming an image.
With improvement in the recent digital drawing technology, attention has been focused on an image record technique for ejecting recording material directly onto a record medium for forming an image without the need for a process of developing, etc. For example, a toner jet record technique is a technique for ejecting powder toner from a recording head to a record medium opposed thereto by the electrostatic force of an electric signal. An ink jet technique of using liquid ink in place of powder toner and ejecting ink droplets from a recording head to a record medium opposed thereto by an external force makes it possible to easily miniaturize a record apparatus because the size of the recording head is small, and is widely in the actual use. The ink droplet jetting techniques include a technique of using piezo elements deformed in response to an electric signal, a technique of using heating resistors for generating heat in response to an electric signal, a technique of using ultrasonic generation elements for generating ultrasonic waves in response to an electric signal, a technique of using the electrostatic force of an electric signal, and the like.
By the way, considering the maintainability of a record apparatus and the transportability of record media, it is desirable that the spacing between a recording head and a record medium should be widened. However, in the technique of using piezo elements, the technique of using heating resistors, and the technique of using ultrasonic generation elements, the jet speed is slow and if the spacing between the recording head and the record medium is widened, the hit position becomes unstable; this is a problem. If the spacing between the recording head using the electrostatic force and the record medium is widened, a high-voltage pulse signal becomes necessary and consequently a control section becomes very expensive; this is a problem.
To solve such problems, a method of using powder toner or liquid toner containing charged matter as a recording material and installing an intermediate electrode between a recording head and a record medium for controlling flying of ejected charged matter is known.
FIG. 5 shows the configuration wherein an intermediate electrode 3 is placed between a recording head 1 and a record medium 5. In FIG. 5, a toner ejection part 11 is opened in a part of the top face of the recording head 1 and an ejection part electrode 2 is placed in the proximity of the ejection part 11 in the outer peripheral portion of the surface of the ejection part 11. A rear electrode 4 on which the record medium 5 is placed is disposed opposed to the recording head 1. The intermediate electrode 3 is disposed between the ejection part electrode 2 and the rear electrode 4 and a stable electric field is formed between the recording head 1 and the intermediate electrode 3 regardless of the distance between the recording head 1 and the record medium 5. As the voltages applied to the electrodes, for example, if the toner (charged matter) particles ejected from the ejection part 11 are positively charged, 0 V is applied to the rear electrode 4, 500 V to the intermediate electrode 3, and 1000 V to the ejection part electrode 2.
The toner ejected from the ejection part 11 of the recording head 1 is first accelerated by an electric field formed by the ejection part electrode 2 and the intermediate electrode 3 (see equipotential line 50) and ejecting of the toner is controlled. Further, the toner passing through the intermediate electrode 3 is accelerated by an electric field formed by the intermediate electrode 3 and the rear electrode 4, is ejected toward the rear electrode 4, and is hit at a predetermined position on the record medium 5.
As described above, as compared with the applied voltage for only the ejection part electrode 2 and the rear electrode 4 to form the electric field for accelerating the particles of the toner ejected from the ejection part 11 and controlling flying, the intermediate electrode 3 is placed between the ejection part electrode 2 and the rear electrode 4 and an electric field is formed by the ejection part electrode 2 and the intermediate electrode 3, whereby the electric field for accelerating the toner ejected from the ejection part 11 and controlling flying can be formed at lower applied voltage and a power unit can be miniaturized.
In the technique of using piezo elements, the technique of using heating resistors, and the technique of using ultrasonic generation elements as the external force for ejecting the charged matter by controlling flying of the toner of the charged matter ejected from the ejection part 11 by the electric field formed by the intermediate electrode 3, if the spacing between the recording head 1 and the record medium 5 is widened, the hit position becomes stable, making it possible to conduct a good image formation as the toner is accelerated by the electric field formed by the ejection part electrode 2 and the intermediate electrode 3.
As described above, the image forming apparatus for ejecting the charged matter directly to the record medium 5 for forming an image comprises the intermediate electrode 3, so that good electric field formation and miniaturization of the apparatus can be accomplished. However, if the intermediate electrode 3 is provided solely, anew dielectric (base material 31 of the intermediate electrode) exists between the recording head 1 and the record medium 5, a steep potential gradient from the ejection part 11 to the intermediate electrode 3 occurs, an electrostatic force acts on the toner along the electric field line (see the equipotential line 50) diverged in the direction of the intermediate electrode 3 from the ejection part 11 of the recording head 1, and the toner ejecting direction is disordered relative to the electric field line direction. Thus, a high-quality image cannot be formed on the record medium 5; this is a problem.